Electron beam furnaces, also termed electron beam evaporators, have heretofore been employed in applications in which it is desired to vaporize a solid material. In particular, in the field of isotopic enrichment of uranium, it has previously been recognized, as shown for example in U.S. Pat. Nos. 3,939,354, and 4,058,667, that a beam of electrons may be impacted upon a reservoir containing a stock of uranium. An electron gun, comprising in part a heated filament, is employed as the source of the electron beam. The electron beam is directed onto and heats the uranium in the region of the beam impact and melts a portion thereof, to create a molten pool of uranium of limited extent. Such localized heating limits the region of melting and permits the remainder of the material in the reservoir to act as a heat sink for the molten pool. Evaporation from the surface of the liquefied pool provides a source of uranium vapor. The vapor will contain multiple isotopes of uranium and, by processes such as those disclosed in the aforementioned patents, desired isotopes may be selectively excited and collected therefrom.
Since vaporization will deplete the stock of uranium acted upon by the electron beam, it is necessary to either continuously or from time to time replenish the supply of uranium in the reservoir. One previous method of doing so on a more or less continuous basis has been to suspend feedstock for replenishing the uranium supply above the material being vaporized, and then to heat the feedstock and allow it to slowly drip into the vapor source supply. This, of course, requires a mechanism for heating and melting the feedstock. In the field of isotopic enrichment, the principal manner of melting the feedstock has heretofore been to place the feedstock in such a position as to intercept a portion of the primary beam of electrons. Unfortunately, this causes the feedstock to spatter, rather than to drip cleanly. A selected isotope of the vaporized material is collected in enriched proportions by separately condensing particles having the isotope on cooled surfaces disposed above the evaporator. Since such collection surfaces are situated in direct line of sight of the reservoir, the spattering results in unprocessed material being collected on these surfaces, thereby decreasing the isotopic selectivity of the separation and the amount of enrichment otherwise achievable.
Alternatively, heat for melting the feedstock has been derived in other prior art systems by radiation from the heated, molten pool. This, however, creates a cold spot in the pool in the vicinity of the feedstock; the result is an undesirable non-uniformity or perturbation of the vapor distribution emanating from the evaporator.